This invention relates generally to gas turbine engines and, more particularly, to methods and apparatus for providing non-equal torque between counter-rotating gas turbine engine rotors that have one or more interdigitated rotors.
At least some known gas turbine engines include a forward fan, a core engine, and a power turbine. The core engine includes at least one compressor, a combustor, a high-pressure turbine, and a low-pressure turbine coupled together in a serial flow relationship. More specifically, the compressor and high-pressure turbine are coupled through a high-pressure shaft to define a high-pressure rotor. The compressor compresses air entering the core engine that is then mixed with fuel and ignited to form a high energy gas stream. The gas stream flows through the high-pressure turbine, rotatably driving it and the high-pressure shaft that, in turn, rotatably drives the compressor.
The gas stream is expanded as it flows through the low-pressure turbine. The low-pressure turbine rotatably drives the fan through a low-pressure shaft such that a low-pressure rotor is defined by the fan, the low-pressure shaft, and the low-pressure turbine. At least some known low pressure turbines include counter-rotating turbines that power counter-rotating fans and counter-rotating boosters and/or low pressure compressors.
Improved engine efficiency and power output may depend on increased flexibility of design choices of each engine component. Specifically, the design of a low-pressure turbine may restrict available design choices for fans and/or boosters to be coupled to the low-pressure turbine. For example, at least some known low-pressure turbines include outlet guide vanes, which may be used to limit the tangential momentum of the combustion gas stream exiting the engine. Removing the outlet guide vanes decreases the overall engine weight, but may increase the detrimental effects of tangential momentum.
Generally, operating known counter-rotating turbines when torque is split substantially equally between the forward and aft shaft shafts facilitates optimizing the efficiency of such turbines. However, improved engine performance may be achieved, for example, by operating the forward fan at a higher fan pressure ratio and/or higher rotational speed than the aft fan, providing boosters or flades (fan-on-blade) to the forward or aft fan shaft, or providing two rotors with an intervening stator on one shaft.
Such operation, however, may result in a substantial non-equal torque demand between the counter-rotating rotors, such as, approximately 2:1 or greater. Such non-equal torque has not been attainable with known counter-rotating low-pressure rotor configurations.